The invention relates to a process for producing a shaped body from metal foam.
EP 0 804 982 A2 has disclosed a process for producing shaped parts from metal foam, in which the starting material used is a compacted mixture of gas-releasing blowing agent and metal powder. A compacted mixture of this type is usually produced by extrusion. The compacted mixture may be in the form of rods, tubes or granules. To produce a metal foam, the compacted mixture is heated in a heatable chamber until the metal melts and the blowing agent decomposes. The gas which is then released causes the metal to foam. The metal foam which is formed is then pressed from the chamber into a casting mold by a plunger.
DE 197 34 394 A1 has disclosed a process in which, to produce a shaped part from a metal foam, the starting point is once again a compacted semifinished product. According to one alternative, it is also possible for the starting material used to be metal which cannot be foamed and for gas or a blowing agent to be supplied separately to the melt in order to form a metal foam.
It is known from DE 42 06 303 C1 to produce shaped bodies from metal foam by starting from a compacted starting material which is produced by a continuous extrusion process and is then comminuted.
DE 197 44 300 A1 has disclosed a further process for producing a shaped body which is formed from a metal foam. In this case, a semifinished product which is produced from a blowing agent which releases gas and a metal powder is heated in a receptacle. The metal foam which forms passes into a casting mold connected downstream.
The abovementioned processes disadvantageously require the expensive and time-consuming production of a compacted starting material or semifinished product. This usually requires the provision of a special device, for example an extrusion press.
DE 1 164 102 has disclosed a further process for producing shaped bodies from metal foam. In this case, a metal melt is fed to a mixer, where it is mixed with a gas-forming substance. To produce the metal melt, the process requires a separate melting device to be provided.
U.S. Pat. No. 5,865,237 has disclosed a process for producing shaped bodies from a metal foam. In this process, a heatable chamber is provided, which is connected to a casting mold. A compacted mixture which is accommodated in the heatable chamber, has been produced by powder metallurgy and contains the metal and a blowing agent is heated until a metal foam is formed. The metal foam is then forced into the casting mold.
The structure of the shaped body is dependent on the filling of the chamber, on the quality of mixing of the powder with the blowing agent and on the heating of the powder in the chamber. These parameters cannot always be set optimally and reproducibly. To ensure that as little blowing agent as possible is lost and to allow economic operation, on the one hand rapid heating is favorable. On the other hand, rapid heating leads to an uneven temperature distribution and to an uneven cell structure of the shaped body. The components which are produced using the known process have irregularities in their structure.
It is an object of the invention to eliminate the drawbacks of the prior art. The intention is, in particular, to provide a process with which shaped bodies comprising metal foam of constant quality can be produced as easily and inexpensively as possible.
This object is achieved by the features described in claim 1. Advantageous configurations result from the features described in claims 2 to 19.
The invention provides a process for producing a shaped body from metal foam, comprising the following steps:
a) providing a first powder, which is formed from the metal, and a second powder, which is formed from a blowing agent,
b) feeding the first and second powders to an extrusion device, the first and second powders being in non-compacted form,
c) conveying a powder mixture, which is formed from the first and second powders, in the extrusion device toward a casting mold, during which step it is at least partially melted and a pressure is applied to the at least partially molten powder mixture, this pressure being greater than a gas pressure produced by the blowing agent,
d) injecting the at least partially molten powder mixture into the casting mold, and
e) relieving the pressure to a level which is lower than the gas pressure, so that the casting mold is completely filled with a metal foam which forms.
In the process according to the invention, non-compacted powder is fed directly to an extrusion device, where it is mixed and at least partially melted. The expensive and time-consuming step of producing a compacted starting powder is avoided. There is no need to provide a special device for producing the compacted mixture.
The proposed process achieves intimate mixing of the at least partially molten powder mixture. Furthermore, in the extrusion device it is possible, without major additional outlay, to apply a pressure which counteracts undesirable premature decomposition of the blowing agent in the extrusion device. The heating of the powder mixture can be accurately controlled along the conveying path. Since the at least partially molten powder mixture is only expanded after it has emerged from the extrusion device into the casting mold, the formation of the metal foam is shifted into the casting mold. The formation of an irregular cell structure is avoided. The shaped bodies produced can be manufactured with a constant quality.
The first powder advantageously has a mean particle diameter in the range from 50 to 250 xcexcm, preferably of 100 xcexcm. The second powder may have a mean particle diameter in the range from 5 to 20 xcexcm, preferably of 10 xcexcm. According to a further design feature, there is provision for the first and second powders to be mixed before they are fed to the extrusion device. The first and/or the second powder may be supplied to the extrusion device under an inert gas atmosphere. This avoids undesirable oxidation of the powder. The quality of the components and their reproducibility are increased.
The first and second powders are expediently mixed in the extrusion device under the action of shear forces. The mixing and/or conveying of the powder mixture can be carried out by the rotary movement of a screw of the extrusion device. In this case, shear forces are also applied to the at least partially molten powder mixture. Undesirable growth of dendritic crystals is avoided. Approximately 100 revolutions per minute represents an advantageous rotational speed of the screw.
The powder or powder mixture is, advantageously heated continuously along a conveying path which extends from a feed opening toward an antechamber. Along the conveying path, the powder mixture is at least partially converted into a foamable melt in a single process step. In the process, both the powder mixture and the at least partially molten powder mixture are being continuously mixed. Furthermore, the powder mixture is conveyed toward an antechambers [sic] which accommodates the at least partially molten powder mixture. The process can be carried out at low cost using a single extrusion device. The separate production of a compacted starting material by extrusion, the subsequent comminution of the extruded semifinished product which may then be required, and the operation of transferring the compacted starting material into the extrusion device are eliminated.
In the extrusion device, the powder mixture is heated to a temperature which is higher than the solidus temperature. It is advantageous for the melt to be heated in the extrusion device to a temperature which is at most 50xc2x0 C., preferably 20xc2x0 C., above the liquidus temperature. It has proven particularly advantageous for the powder mixture to be heated to a temperature which lies in the range between the solidus temperature and the liquidus temperature. In this case, the powder mixture is only partially melted. The at least partially molten powder mixture may have a solid-phase content of 20 to 50%, preferably 30 to 40%. The at least partially molten powder mixture is expediently accumulated in a semi-solid thixotropic state in the antechamber. In the partially molten state, the viscosity is considerably increased compared to the completely molten state. This has the advantageous result that a continuous melt front is formed during injection into the casting mold. This means that undesirable atomization of the material which is injected into the casting mold and premature escape of the gas formed by the blowing agent are avoided. On account of the pressure relief in the casting mold, complete melting of the injected material is nevertheless achieved. In the case of injection of an only partially molten powder mixture, however, the release of the gas formed from the blowing agent is delayed. In this case, shaped bodies with a homogeneous foam structure are formed, i.e. in particular the formation of giant bubbles is prevented.
To keep the gas pressure which is generated as low as possible, it is expedient for the blowing agent used to be a pre-oxidized blowing agent.
The powder mixture can be heated in the extrusion device by means of an external heater device, for example by means of external strip heaters or an induction device. A heater device of this type allows accurate setting of the heating rate of the powder mixture in the extrusion device. This allows premature decomposition of the blowing agent and therefore expansion of the melt to be avoided. Furthermore, the temperature can be set in such a way that undesirable growth of dendritic metals is avoided. The build-up of a suitable pressure can be controlled using known mechanical and process engineering measures. It is expedient for a pressure of more than 10 bar, preferably of more than 30 bar, to be applied in the extrusion device above a temperature of more than 300xc2x0 C.
The injection of the at least partially molten powder mixture can be effected by an axial movement of thee screw directed toward the casting mold. The extrusion device is expediently provided with a mechanical valve to optionally open and close an antechamber arranged downstream of the screw. Suitable devices are known, for example, from U.S. Pat. No. 5,040,589 or EP 0 409 966 B1, the disclosure of which is hereby incorporated.
According to a further design feature, the casting mold is preheated. This avoids excessively rapid solidification of the melt in the region of contact with the casting mold. To carry out the proposed process, the metal used is expediently magnesium or a magnesium alloy.
According to a further design feature, there is provision for a mold cavity which is surrounded by the casting mold to be increased in size after the injection. In this case, the casting mold used is expediently a positive mold. With a mold of this type, it is possible for at least one wall of the casting mold to be moved in the manner of a plunger, so that the size of the mold cavity is increased.
The metal used may be magnesium, aluminum, a magnesium alloy or an aluminum alloy. The blowing agent used may be a metal hydride, preferably TiH2 or MgH2. The blowing agent usually forms 0.5% by weight of the total weight of the powder.